The New Madrid Seismic Zone (NMSZ) is a fault system spanning approximately 240 kilometers across Illinois, Arkansas, Tennessee, Kentucky, and Missouri. The paleoseismic history, ongoing seismic activity, and high population density make the NMSZ a key region for seismic research focusing on fault activity and, ultimately, improving community preparedness.

Large earthquakes may be few and far between, but the zone is constantly active and undergoes diverse forms of movement, including “creep”—the ongoing fault displacements that are too subtle to constitute an earthquake. Specifically, afterslip describes creep that occurs along a fault after it has ruptured in a large seismic event. Identifying the source of present-day crustal strain that will lead to future earthquakes is a vital step for improving awareness of seismic hazards and how they evolve.

Here Boyd et al. investigate such activity by analyzing GPS data from monitors across the NMSZ collected between 2000 and 2014. The team modeled the data for a range of deformation mechanisms—processes by which strain accumulates in materials at the grain scale—including steady subsurface creep and regional strain, or prolonged rock deformation, and time-dependent afterslip and viscoelastic relaxation resulting from previous earthquakes.

The region has been the source of several major earthquakes over the past thousand years, including three events in the winter of 1811–1812 that topped magnitude 7. Previous studies have suggested that the Reelfoot fault was responsible for a large quake in 1450 and one of the 1812 events, but other large events are attributed to the Cottonwood Grove fault and other nearby faults.

Although debate has been ongoing for more than a decade about whether there is significant strain in the NMSZ, whether present-day activity represents a long-lived aftershock sequence, and whether the NMSZ will continue to produce damaging earthquakes in the near future, the authors argue that current surface deformation can be partly explained by afterslip from the 1450 and 1812 Reelfoot events and suggest that much of the recent activity in the NMSZ may be the result of a long-lived aftershock sequence from previous large earthquakes.

The authors stress that this conclusion does not erase the possibility that the NMSZ is still in an active phase and will continue to produce new, powerful earthquakes over relatively short recurrence intervals. This study is an argument for the continuing threat of high seismic hazard due to the New Madrid Seismic Zone and for the critical importance of resilient infrastructure and adequate preparation. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2015JB012049, 2015)

—Lily Strelich, Freelance Writer

Citation: Strelich, L. (2015), Aftershocks of old quakes still shake New Madrid Seismic Zone, Eos, 96, doi:10.1029/2015EO040129. Published on 24 November 2015.

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